1. Field of the Invention
The invention generally relates to integrated circuits and more particularly to radio-frequency integrated circuits designed to operate at frequencies of 1 GHz and above.
2. Description of Related Art
Radio-frequency integrated circuits include various functional units within the same substrate, for example low-noise amplifiers (LNA), local oscillators, power amplifiers and phase-locked loops.
One of the major problems to be solved in designing radio-frequency circuits is to minimize the risk of coupling between the various units. This is because any coupling from one unit to another can cause a malfunction of the integrated circuit, as in the case of coupling between a low-noise amplifier of a receive channel and a power amplifier of a send channel, for example. This is because coupling is then created between the input and the output of the circuit, which causes a malfunction of the circuit.
There are currently many causes of coupling between units. For example, coupling between units can result from interconnection problems or from design errors in the layout of the electronic components. Coupling can also occur via the silicon substrate of the integrated circuit.
The silicon substrate is inherently conductive. The various grounds of the integrated circuits are therefore connected via the resistance of the substrate.
To minimize, or even eliminate, coupling between units via the substrate, it has already been proposed to place insulative walls in the substrate between the various functional units.
However, there is another problem, previously unidentified, and which is also the cause of coupling between the various functional units of an integrated circuit via the substrate.
This problem lies in the presence of a seal ring around the electronic components of the integrated circuit, which is inherent to the technology.
The seal ring prevents pollution of the integrated circuit by polluting particles resulting from cutting the semiconductor wafer along cutting paths delimiting the various integrated circuits of the wafer.
The seal ring is in fact a wall made up of all the layers available in the integrated circuit as well as a caisson formed in the substrate, for example a P+-type caisson.
Because of its nature, and because it has a higher conductivity, the seal ring constitutes a low-impedance path. Consequently, if an interfering signal is emitted into the substrate by a functional unit located near the seal ring, that interference signal will naturally take the low-impedance path consisting of the seal ring and propagate along the ring, possibly until it creates unwanted coupling with another functional unit in another part of the integrated circuit and also beside the seal ring.
Accordingly, a need exists to overcome the problems in the prior art and to provide a integrated circuit with reduced coupling.